1. Field of the Invention
The present invention relates to a power supply device for electric equipment of an automotive vehicle, such as an engine starter.
2. Description of the Prior Art
FIG. 1 shows a conventional power supply device for an engine starter as an example of a power supply device for electric equipment of an automobile. The power supply device comprises a battery 1 coupled to a starter 2 and a switch 3 at the positive terminal thereof. The battery 1 is grounded at the negative terminal thereof. The starter 2 comprises an electromagnetic switch 21 and a starter motor 22. A fixed contact 21a of the switch 21 is coupled to the positive terminal of the battery 1, while the other fixed contact 21b is coupled to a field magnet coil 22a of the motor 2. A movable contact 21c is moved by an excitation coil 21d coupled to a relay 9. The armature 22d of motor 2 is supplied with electric current through a brush 22b coupled to the field magnet coil 22a and a grounded brush 22c. A constant voltage source circuit 6 coupled to the battery 1 at the positive terminal thereof supplies an operating voltage of perdetermined level to a control circuit 7, which, in response to a starting signal from the switch 3 and an engine rotation signal from an engine rotation sensor 8, controls the making and breaking of relay 9, which includes a relay coil 9a and a usually open conact 9b operated by it.
The operation of the device of FIG. 1 is as follows. When the starter switch 3 is made, control circuit 7 supplies a voltage to relay coil 9a, thereby closing the contact 9b to apply the battery voltage E to the excitation coil 21d. Thus, the movable contact 21c comes into contact with the fixed contacts 21a and 21b to supply the battery voltage E to the starter motor 22. When the starter motor 22 has started the associated engine, the rotation sensor 8 generates a rotation signal, and in response thereto, control circuit 7 stops supplying a voltage to the relay coil 9a to open the contact 9b. As a result, the electromagnetic switch 21 is opened and the starter 2 is stopped.
The conventional power supply device shown in FIG. 1 has following disadvantages. Namely, the voltage E across the positive and negative terminals of the battery 1 varies as shown in FIG. 2: When the switch 3 is made to supply voltage E to the starter 2 at a time point A, a rush current flowing through the starter 2 causes the battery voltage E (which has been at a constant rated voltage E0 before time point A) to fall abruptly to a minimum E1 at a time point B. Thereafter, the voltage E gradually increases as the rotational speed of the starter increases. The voltage drop (E0-E1) due to the rush current is especially severe when the ambient temperature is low: In such cases, the minimum voltage E0 may become as low as 4 V. On the other hand, IC (integrated circuit), etc., utilized in control circuit 7 generally operates at a voltage level of about 5 V. Thus, constant voltage source circuit 6 is designed to apply 5 V to control circuit 7 when it is in proper operation. To maintain the output voltage of constant voltage source circuit 6 at 5 V, the input voltage supplied thereto should be kept within the range of from 6 to 20 V. Thus, when the minimum voltage E0 becomes lower than the lower limit (i.e. 6 V) of the acceptable input voltage range (i.e., 6 to 20 V) of constant voltage source circuit 6, the output voltage thereof applied to control circuit 7 becomes lower than the operating voltage (5 V) of control circuit 7. As a result, relay 9 is turned off before the enginne is started. If this happens, it becomes impossibe to start the engine.
In view of the disadvantage of the conventional power supply device of FIG. 1, Japanese laid-open patent application No. 59-155550 proposes to suppress the operation of the control circuit for a predtermined time interval during the starting of the engine, for the purpose of preventing the above-mentioned malfuctioning of the control circuit which may be caused by the lowering of the battery voltage. This suppression of the control circuit, however, results in a loss of control of the power supply circuit, even if for a short period during the starting of the engine. Thus, troubles may ensue in this period.
Alternatively, provision of a back-up battery or a capacitor of a large capacity for supplying the control circuit with power may be contemplated for the purpose of compensating for the lowering of the battery voltage at the starting of the engine. These measures, however, also have problems with regard to maintenance and durability thereof.